Prepare rice according to package directions; set aside.
Melt butter in a large skillet over medium heat; add onions, celery, and water chestnuts.
Sauté 10 minutes or until vegetables are tender.
Stir in rice, chicken, 3 cups Cheddar cheese, and next 5 ingredients; spoon mixture into a lightly greased 15- x 10-inch baking dish or a 4-quart casserole.
Top with breadcrumbs, if desired.
Bake casserole at 350° for 30 minutes.
Sprinkle with remaining 1 cup Cheddar cheese, and bake 5 more minutes.
To Make Ahead: Casserole may be frozen up to 1 month.
Let stand at room temperature 1 hour.
Bake, covered, at 350° for 30 minutes.
Uncover and bake 55 more minutes.
Sprinkle with cheese, and bake 5 more minutes.